Noting the continued fast progression of wireless communication technologies, wireless communication products are increasingly widely used in our daily lives. The antenna is one of the most important devices for wireless communication products. In general, the antenna may occupy considerable space in wireless communication products. Thus, the reduction of antenna size in order to minimize the size of communication products is a large issue and of great concern.
A microstrip antenna usually refers to an antenna fabricated through the use of microstrip techniques. Microstrip antennae have attracted a great deal of attention due to their plane structure, low cost, ease to be mass produced, and integrated with active components or circuit boards. Thus, microstrip antennae have been widely used in many portable devices that require wireless communication capability, such as mobile phones, smart phones, tablet computers, notebook computers, GPS (Global Positioning System) units, and RFID devices. Despite the many advantages of microstrip antennae, they have several basic disadvantages, such as narrow bandwidth, low gain, and relatively large size.
With the aim of reducing the size and weight of portable computers, the PIFA (Planar Inverted F Antenna) or monopole antenna can be set up on the circuit board of the portable device. However, because of rapid developments in the wireless communication industry, most mobile devices are installed with communication modules that must transmit or receive signals at various frequency bands. Therefore, antennae with multiple resonance frequencies are essential elements in most mobile devices. To design a monopole or PIFA antenna with multiple resonance frequencies, the area or space of a circuit board needs to be large. In actual application, in order to meet the requirement of at least a quarter of the wavelength, the dimensions of the monopole or PIFA antenna cannot be further reduced.